The present invention relates to steam turbine systems and, more particularly, to an improved turbine cycle arrangement using a turbine drive for a boiler feedwater pump.
In the mid-1950's it was proposed to develop a single turbine drive for the boiler feedwater pump in a steam turbine system in order to improve cycle efficiency. Since that time, a number of boiler feed pump turbine (BFPT) drive/feedwater train arrangements have been implemented. Using a steam turbine to drive the main feedwater pump improves cycle efficiency because of the variable speed capability of the turbine. The initial applications, on single reheat subcritical plants, used cold reheat steam as the energy supply to a non-condensing BFPT. As the concept evolved, the BFPT supplied one or more feedwater heaters and usually exhausted to the deaerator, at which point there was also a connection to the intermediate pressure (IP) exhaust of the main unit. FIG. 1 illustrates a typical prior art arrangement of BFPT 10 in a steam turbine power system 12. The system 12 includes a high pressure (HP) turbine 14 and an intermediate pressure (IP) turbine 16 coupled in driving relationship to an electric power generator 18. A low pressure (LP) turbine 20 is coupled to drive another electric power generator 22. A boiler 24 supplies steam to drive the turbines. A plurality of feedwater heaters 26A-26F utilize steam extracted from the turbines to reheat water collected at condenser 28 and pumped back to boiler 24. At high main unit load, the BFPT exhaust 30 and the main unit IP exhaust 32 share in the steam demand of the deaerator 34. At lower loads, the BFPT exhaust 30 alone supplies the deaerator 34. At still lower loads, there was excess steam at the BFPT exhaust and not only did it supply the deaerator but excess steam was also sent back to the IP exhaust 32. There is a sizable difference in temperature between the BFPT exhaust steam and the IP exhaust steam. In the typical installation illustrated by FIG. 1, the difference in steam temperatures is about 180.degree. F. at maximum load and increases to about 240.degree. F. at 35% load when BFPT steam is sent to the IP exhaust.
In other BFPT applications, the main turbine IP exhaust and the BFPT connect to a common heater at an upstream location in the BFPT. In this instance, the BFPT exhaust and its associated heater "float". In FIG. 2, for example, the tie-in with the main unit (IP exhaust) occurs at heater 26E. The BFPT alone supplies heater 26D and 26F. The difference in steam temperature between the two sources for heater 26E is about 290.degree. F. at maximum load and increases to about 350.degree. F. at 50% load.
Various other arrangements of BFPT's have been tried, including an arrangement on a double reheat turbine where the three lowest pressure heaters, e.g., heaters 26A, 26B and 26C of FIG. 3, receive steam from the BFPT. In such an arrangement, heater 26C is connected to the second blade group exit of the LP turbine as well as the highest pressure extraction point in the BFPT 10. In other systems, non-condensing BFPT arrangements have been superseded by applications in which a straight condensing BFPT is used. In these systems, the BFPT does not supply any feedwater heaters and receives steam from the crossover pipe to the LP turbine. An example of a condensing BFPT application is shown in FIG. 4. The BFPT 10 receives steam from the IP turbine exhaust and exhausts its steam to the condenser 28.
With the application of double reheat cycles and reheat temperatures above 1000.degree. F., the difference between the extraction steam temperature and the saturation temperature in the feedwater heaters increases considerably as shown in the graphs of FIG. 5. As the temperature difference increases, there is an increase in the loss of available energy during the heat transfer process on cycles which use a condensing BFPT. Of special concern is the high steam temperature at the first extraction point after the second cold reheat. During cycle optimization studies of a 1000 MW double reheat turbine (steam conditions of 4500 psig, 1100.degree. F./1100.degree. F./1100.degree. F.), the steam temperature was 955.degree. F. for the heater supplied from the first extraction point in the IP turbine (after the 2nd reheat). This is about 30.degree. F. higher than the maximum load impulse chamber (first stage exit in the HP turbine) temperature with typical 2400 psig, 1000.degree. F. and 3500 psig, 1000.degree. F. main steam conditions. In addition, the steam temperature at the next two extraction points were 760.degree. F. and 615.degree. F. which are considerably above the temperature where carbon steel extraction piping would be used. So at least two and possibly other extraction steam lines and their respective heaters (shells, tubes and other internals) would require alloy materials. Piping design, to avoid excessive reactions, would also be more complicated and costly.
In a computer simulation, a condensing BFPT system using a double reheat turbine was modified to use a non-condensing BFPT 10A in the manner shown in FIG. 4A. Compared to FIG. 3, the two heaters 26E, 26F that had been supplied by the IP (2nd reheat) turbine 16 are now coupled to the BFPT 10A. The BFPT 10A is also coupled to supply the heater 26D that had been supplied from the LP turbines 20 and exhausted to the next lower pressure heater 26C, which is also coupled to an extraction point on LP turbines 20. The BFPT exhaust flow was greater than this lower pressure heater 26C could condense, so the excess was returned to the first group exit of the LP turbines 20. The temperature of the BFPT exhaust steam was 285.degree. while the LP turbine steam temperature was 450.degree. F. or a difference of 165.degree. F. There was a heat rate improvement of 0.12% with the non-condensing BFPT as compared to the condensing BFPT cycle. This difference also included a decrease in BFPT blading efficiency as compared to the condensing drive. Even if there were no heat rate improvement, the cost savings related to the extraction piping and feedwater heaters would reduce the plant capital cost. In addition, the second reheater size and reheat piping would be reduced because of reduced reheater mass flow. Even with the above discussed modifications, there is a concern about the 165.degree. F. difference in temperature between the steam in the LP turbines and the steam returning from the heater supplied by the BFPT exhaust. Moreover, this difference in temperature would increase as main unit load is reduced. Accordingly, it is desirable to provide a system in which cold BFPT steam will not contact hot LP turbine parts.